The present invention relates to a flexible pipe for carrying hydrocarbons or other fluids at high pressure, and to a method of producing such a pipe. More specifically, the invention relates to a flexible pipe that has high resistance to axial compression (axial compression strength).
Flexible pipes for carrying hydrocarbons are already well known and generally comprise, from the inside of the pipe outward, a metal carcass, to react to radial crushing forces, covered by an internal sealing sheath made of polymer, a pressure vault to withstand the internal pressure of the hydrocarbon, tensile pressure armor layers to react to axial tensile forces and an external sheath made of polymer to protect the entire pipe and, in particular, to prevent the ingress of seawater into its thickness. The metal carcass and the pressure vault are made of longitudinal elements wound with a short pitch, and give the pipe its ability to withstand radial forces, while the tensile pressure armor layers consist of metal wires wound with long pitches in order to react to axial forces. It should be noted that, in the present application, the idea of winding at a short pitch denotes any helical winding at a helix angle of close to 90°, typically ranging between 75° and 90°. The idea of winding at a long pitch for its part covers helix angles of below 55°, typically ranging between 25° and 55° in the case of tensile pressure armor layers.
These pipes are intended to carry hydrocarbons particularly along the seabed and to do so at great depths. More specifically, they are of the kind known as “unbonded” and are thus described in the Standards published by the American Petroleum Institute (API) API 17J and API RP 17B.
When a pipe, whatever its structure, is subjected to an external pressure that is higher than the internal pressure, compressive forces directed parallel to the axis of the pipe are generated in the wall of the pipe and have a tendency to shorten the length of the pipe. This phenomenon is well known by its English name of “reverse end-cap effect”. The intensity of the axial compressive forces is proportional to the difference between the external pressure and the internal pressure. This intensity may reach very high levels in the case of a flexible pipe submerged at a great depth because the internal pressure may, under certain conditions, be very much lower than the hydrostatic pressure.
In the case of a flexible pipe of conventional structure, for example one in accordance with the API standards, the reverse end-cap effect has a tendency to introduce a longitudinal compressive force into the wires that make up the tensile pressure armor layers and to shorten the length of the flexible pipe. In addition, the flexible pipe is also subjected to dynamic loading particularly when it is installed or in use as what is commonly known in English as a “riser”. All of these stresses may cause the wires of the tensile pressure armor layers to buckle and irreversibly disorganize the tensile pressure armor layers, thus destroying the flexible pipe.
Document WO 03/083343 describes a solution for increasing the axial compression strength of the tensile pressure armor layers of a flexible pipe. This solution consists in winding around the tensile pressure armor layers tapes that are reinforced, for example, with aramid fibers. This limits and controls the expansion of the tensile pressure armor layers. However, while this solution does solve the problems associated with the radial buckling of the wires that make up the tensile pressure armor layers, it merely lessens the risk of lateral buckling of said wires, which risk still remains.
Document WO 03/056224 describes a solution for reducing the risk of lateral buckling of the wires that make up the tensile pressure armor layers of a flexible pipe subjected to an axial compressive force. This solution consists in reducing the lateral clearances between wires and optionally in filling said clearances with a filling material. However, while this solution reduces the risk of lateral buckling it does not completely eliminate this risk. In addition, that solution has the disadvantage of significantly increasing the complexity and cost of manufacture of the tensile pressure armor layers, on account in particular of the tighter dimensional tolerances.
Document WO 2006/042939 also describes a solution for reducing the risk of lateral buckling. That solution consists in using wires that have a high width-to-thickness ratio and in reducing the total number of wires that make up each tensile pressure armor layer. However, while that solution reduces the risk of lateral buckling of the tensile pressure armor layers it does not completely eliminate it.
Document WO 01/81809 describes a solution that consists in producing the pressure vault of the pipe from K-shaped interlocked wires, and in using said pressure vault as a mechanical end stop to react to axial compressive forces. In addition, the tensile pressure armor layers are free to expand because there is no external sealing sheath nor is there any reinforcing layer capable of restricting the extent to which they expand. When a pipe such as this is subjected to an axial compressive force, it shortens until such point as the axial clearances separating the coils of the pressure vault become zero and said coils come into abutment against one another, in which configuration said pressure vault is able to react to most of the axial compressive force. The tensile pressure armor layers accommodate the shortening by expanding, and make practically no contribution toward reacting to the axial compressive force. In practice, the shortening of such a pipe is generally great, typically of the order of 5% of its length. This order of magnitude is a direct result of the geometry of the pressure vault and, more particularly, of the ratio between, on the one hand, the combined length of the axial clearances separating the coils and, on the other hand, the overall length of the pipe. This is an indirect result of the general design rules disseminated by the API Standards, said rules being aimed amongst other things at minimizing the bend radius at which the pipe can be bent without suffering damage, this being with a view to making handling and storage operations easier. Now, the fact that such a pipe can shorten so substantially when subjected to a reverse end-cap effect, presents a number of problems. First, this shortening causes a significant expansion of the tensile pressure armor layers with a risk of irreversibly disorganizing these, particularly if the pipe is at the same time loaded in dynamic bending. By way of example, a tensile pressure armor layer manufactured with a helix angle of 35° is expanded by almost 10% when shortened by 5%. Under the same shortening conditions, a tensile pressure armor layer manufactured with a helix angle of 25° is expanded by the order of 20%. With such levels of relative expansion, the radial displacements of the wires may be 5 to 10 times greater than their own thickness, which goes some way to explaining the risk of disorganization of the tensile pressure armor layers. Another disadvantage with the potential shortening is that a pipe such as this has a tendency to straighten itself out when subjected to a reverse end-cap effect, thus generating instabilities and bending movements that could have damaging effects particularly at connections with underwater equipment.
Document WO 96/17198 describes, particularly in FIG. 18, a flexible pipe comprising a tubular axial blocking layer able to react to axial compressive forces and to limit the shortening of the pipe, thus avoiding damage to the tensile pressure armor layers.
This tubular axial blocking layer comprises two section wires of trapezoidal shape, wound at a short pitch and resting against one another along their inclined flanks so as to form contiguous coils. The flexibility of this layer lies in the relative radial mobility of the two wires that can slide one along the other along their inclined flanks. This layer is positioned around the internal sealing tube so that it also acts as a pressure vault. However, tests have shown that a pipe such as this presents a risk of damage when simultaneously subjected to a high axial compressive force and to repeated reverse-cycle bending stresses, as may be the case with the bottom part of risers near the point of contact with the sea bed. This damage relates more specifically to the axial blocking layer which may gradually become disorganized and lose all or some of its flexibility, thus causing the pipe to be destroyed.
Hence, one problem that arises and that the present invention addresses is that of providing a flexible pipe which is not only capable of withstanding a great deal of axial compression without shortening, but which is also capable durably of withstanding repeated reverse-cycle bending stresses while at the same time maintaining its flexibility and integrity. Furthermore, it is desirable for this pipe to be able to be bent to small bend radii.